The present invention relates generally to heat transfer systems. More particularly, the invention relates to a small highly reliable, low maintenance, light weight solid state refrigeration system particularly valuable at cryogenic temperatures. Alternatively, the system may be used as a heater.
Many infrared (IR) detectors are operative only at low temperatures. These detectors conventionally are cooled by miniature cryogenic refrigerators which are expensive and have exhibited high failure rates. Because frequent maintenance is also required, e.g., approximately every 200 to 1000 hours, a large number of the refrigeration units are out of service and others remain in questionable condition. A truly reliable and low cost cryogenic refrigerator is therefore needed. Such a refrigerator would solve these and other problems, and would also stimulate research into further development of more sensitive detectors.
IR detection is only one of several areas where cryogenic ambients are required with the components to be cooled being sufficiently small to be serviced by a miniature refrigeration system. Many superconducting devices, such as computer elements, etc., require temperatures in the range of 4K. Lazer cooling and the use of miniature cryo-electric elements for computer memories are areas which still remain relatively unexplored, partly because of limitations in the present methods of cryogenic refrigeration.
Certain rare earth iron garnets possess a large, negative magnetocaloric effect in the temperature range of interest, e.g. 4K to 25K. This temperature range is precisely the range where the cost of existing cryogenic refrigeration systems skyrocket. In addition, the existing 4K units generally require large compressors which increase the weight of the system from 15 pounds to over 400 pounds and input power from 0.5 kW to 5.5 kW. Thus only certain applications are possible with these low temperature units.
In U.S. Pat. No. 3,841,107, issued Oct. 15, 1974, to A. E. Clark, coinventor herein, a magnetic refrigeration system is described which utilizes the aforementioned unique characteristic of the rare earth iron garnet in conjunction with conventional paramagnetic material. A heat source and a heat sink are thermally connected by a serial arrangement of magnetocaloric elements having a large negative magnetocaloric effect in abutting relationship with a paramagnetic material. Elements of a suitable superconducting material functioning as thermal valves, are interdisposed between the heat source, the heat sink and the aforesaid serial arrangement. Movement of a magnetic field from the magnetocaloric element to the paramagnetic element effects the refrigeration cycle.
The improved, cryogenic, heat transfer system described herein is a simple, light weight cryogenic refrigeration or heater particularly applicable for temperatures below 20K. The cost and weight of such a system operating down to 4K is competitive with that of the existing 20K plus systems. The present system contains few or no moving parts and thereby is nearly maintenance free. In contrast to the aforesaid magnetic refrigeration system of Clark which employs thermal valves of superconducting material, i.e., materials which exhibit changes in thermal conductivity in the presence of a magnetic field, thermal flow in the present invention is regulated by materials possessing thermal conductivities which are strongly temperature dependent in the cryogenic temperature range of interest.